When a sealing device having a lip portion and a slinger in sliding contact with the lip portion is used in a bearing, friction loss unavoidably occurs between the lip portion and the slinger. The slinger preferably has a small coefficient of friction on a sliding contact surface thereof so as to effectively use a power. In terms of useful life of the sealing device, additionally, it is necessary to reduce abrasion of the lip portion. To meet these demands, a variety of structures have been proposed (for example, see patent literature 1 below). FIG. 20 hereof shows in cross-section a sealing device 100 for a bearing disclosed in patent literature 1.
As shown in FIG. 20, the sealing device 100 includes a seal member 102 reinforced by a core metal 101, and a slinger 105 in sliding contact with lip portions 103, 104 of the seal member 102. The slinger 105 is L-shaped in cross-section and has a sliding contact surface 106 to which a hard chromium electroplating layer 107 is applied.
The hard chromium electroplating layer 107 can reduce abrasion of the slinger 105, thereby extending useful life of the sealing device. However, it is not possible to improve the coefficient of friction. Patent literature 2 below teaches a structure designed to provide a small coefficient of friction.
Patent literature 2 teaches that the hard chromium electroplating layer 107 is replaced with DLC (diamond-like carbon). Since diamond is hard and carbon is better self-lubricating, DLC is hard and better self-lubricating.
Carbon is self-lubricating enough to prevent seizure even when the sliding contact surface lacks a lubricant such as grease. When the sliding contact surface lacks a lubricant serving as a heat-conducting agent, a large torque is generated due to the lack of grease. At this time, the sliding contact surface becomes high in temperature due to accumulation of heat thereon. The high temperature causes thermal degradation of the seal member. This results in shortened useful life of the seal member. What has been desired is a sealing device for a bearing configured to retain a lubricant on a sliding contact surface of the device for preventing a large torque from being generated due to lack of grease.